When heat is to be exchanged between one or more fluids which are at high temperatures or which are chemically corrosive, the heat exchanger must be constructed of special materials designed to resist chemical corrosion and to remain stable at high temperatures. This is especially true of the core member of the heat exchanger which is that portion of the heat exchanger wherein transfer of heat from one fluid to another occurs. Materials which have been used in the past have included special metals and alloys thereof, as well as carbon in its various forms, including graphite.
Graphite heat exchangers have a number of advantages which make them especially desirable for high temperature, high chemical corrosion usage. For example, graphite withstands thermal shock to a limited extent and is quite resistant to chemical corrosion, with the exception of certain strong oxidizing chemicals. Furthermore, graphite structures have excellent stability at temperatures up to about 350.degree. F., as well as good thermal conductivity. There are certain disadvantages to graphite structures which limit their use in heat exchangers. For example, graphite has relatively low tensile strength, so that tubes made of graphite are relatively fragile. Graphite is also porous and permeable to various fluids. This permeability may be overcome by impregnating the graphite with certain synthetic resins, but this reduces the stability of the graphite-synthetic resin compositions at high temperatures. Furthermore, conventional synthetic graphite is made of randomly oriented crystals which limit the effectiveness with which heat can be conducted. Graphite heat exchangers have been designed to take advantage of the good compressive strength and other advantages of graphite, while at the same time minimizing its disadvantages, such as poor tensile strength. For example, U.S. Pat. Nos. 2,821,369; 2,887,303 and 3,106,957 show graphite heat exchangers wherein the core member is made up of a number of graphite blocks. These blocks are held within a shell in a relationship such that holes bored through the blocks are interconnected in such a manner as to permit the exchange of heat between the fluids passing through the core. In certain instances, these blocks may be glued together so that the core member is essentially a monolithic block. U.S. Pat. Nos. 3,265,124 and 3,327,777 disclose the use of graphite tubes, either composed entirely of graphite or a composite tube containing an inner or outer shell which helps to support the graphite. One thing that each of these designs has in common is that they are expensive to produce and difficult to repair.